notes: OSPF Demand Circuit

- An enhancement which suppresses the hello and LSA refresh functions, so that a link does not have to be constantly up.
- OSPF brings up a demand link up to perform the initial database synchronization and subsequently, to flood only LSAs in which certain changes have occurred.

These LSA changes are :

- A change in the LSA options field.
- A new instance of an existing LSA is received in which the age is MaxAge.
- A change in the Length field of the LSA header.
- A change in the contents of the LSA, excluding the 20-octet header, the checksum, or the sequence number.
- Because no periodic hellos are exchanged (Hellos are used only to bring up the link),
OSPF must make a presumption of reachability.
- Demand circuit must be a point-to-point link for OSPF.
- Command "ip ospf demand-circuit" is only needed on the one side
- Changes to the interface and neighbor state machines and to the flooding procedure:
    - MaxAge = DoNotAge
    - A new flag known as the demand circuit bit (DC-bit) is added to all LSAs it originates
    - The DoNotAge bit is set on LSAs advertised out, the interface and the LSAs are not refreshed, unless they change.

commands:


- Configures the connected interface to the demand-circuit

ip ospf demand-circuit

- The DoNotAge bit is set on LSAs advertised out of the interface

ip ospf flood-reduction

ex. PPPoE
       r1                               r2
10.1.1.1/24 <--------->10.1.1.2./24

network command:
network 10.1.1.0 0.0.0.255 area 0

on the routing table @ r1,  it will have 2 routes because it will produce 2 LSAs.
10.1.1.2/32  --- PPP behavior  peer neighbhor address
10.1.1.0/24 --- -directly connected

3 ways to solve this.
1. interface use  " no peer-neighbor route"

2.  ospf network command use 10.1.1.1 0.0.0.0 area 0

3.  interface command " ip ospf 1 area 0.